An aircraft typically utilizes a number of sensors of different types. Many military aircraft and an increasing number of civilian aircraft have infrared sensors to detect potential threats such as missiles fired at the aircraft. The infrared sensor senses the presence of an infrared signature, such as that produced by the motor of a missile approaching the aircraft, and provides a warning so that countermeasures may be employed.
In one type of infrared sensor system, the sensor (detector) is operated at a reduced temperature to improve its performance. The reduced temperature may be produced in any manner, but it is typically produced by a small cryocooler that is integral with the sensor system inside its sensor housing. The cryocooler is operable to cool the sensor to temperatures of about 77 K or less. In other cases, the sensor may be cooled to about 180 K by means of a thermoelectric heat pump. However, all cryogenic coolers are limited in heat removal rate and in temperature differential. Cryocooler operation is handicapped and may become impossible as the heat load and external temperature increase.
In a typical case, the infrared sensor is a wide-angle sensor aimed in a fixed direction relative to the aircraft. For example, the sensor may be a forward facing sensor, an aft facing sensor, or a side-facing sensor. This arrangement minimizes the weight of the sensor system, by avoiding the use of an aiming system. Such minimization of the weight is highly desirable, because the weight of the infrared sensor must always be carried with the aircraft, even though the sensor system is actually required in some cases only a few times, or in some cases not at all, during the life of the aircraft. Additionally, and very importantly, the overall physical size (“envelope”) of the sensor system must be maintained very small, because the sensor system is usually positioned on the aircraft at a location with an unobstructed view of the intended field of view, but which also has very little space available due to the aircraft's geometry and/or other instrumentation that must be similarly positioned.
The forward-facing sensor system faces directly, or nearly directly, into the airstream as the aircraft flies. Objects moving at high speeds through the atmosphere become heated when moving air comes to rest against the obstacle. For example, the high kinetic energy of the moving air is converted by frictional processes into potential energy in the form of high local temperatures and pressures. The local elevated temperature is termed the stagnation temperature. The value of the stagnation temperature increases sharply with aircraft speed. Even at a subsonic speed of 0.8 Mach, the stagnation temperature is 134° C. when the ambient air temperature is 90° C. The same 134° C. stagnation temperature is reached at 1.4 Mach when the ambient air temperature is a cooler 71° C. These are common military flight conditions.
Consequently, the forward-facing sensor system presents some challenging operational problems in situations where the aircraft flies sufficiently fast, such as at supersonic or near-supersonic speeds. In cases where the sensor system flies at such speeds for an extended period of time, the sensor system may be provided with a cryocooler with sufficient cooling power to maintain the reduced temperature of the sensor system during the extended period. However, where the aircraft attains these high speeds only for a short time, termed a “dash”, sizing the cryocooler with sufficient cooling power for the transient heating that occurs during that short period typically results in significant penalties in weight, size envelope, and power consumption. The sensor system must be maintained operational during the dash, because the dash typically occurs in tactical situations where the sensor system is most necessary.
There is a need for an approach to providing cryocooled sensor systems operated in an elevated-temperature environment, without significantly increasing the weight of the sensor system. The present invention fulfills this need, and further provides related advantages.